1. Field of the Invention
This invention relates to a device and method for detecting the phase of a signal at a known frequency and, more particularly, to a digital phase detector adapted for use with the Omega navigational system.
2. Description of the Prior Art
The Omega navigation system was developed in order to provide a means by which navigators may accurately determine their position from a location at any point on the earth. In operation, the Omega system utilizes eight transmitting stations which are located at points spaced over the entire earth. Each of these transmitting stations broadcasts signals at a very low frequency (VLF), on the order of ten kHz. Consequently, these signals have an effective range of five to eight thousand miles. With such a range, the signals from several of the eight Omega transmitting stations should be receivable at any location worldwide.
Each Omega station transmits a continuous wave signal at a known frequency within a discrete time interval. Each station's signal is synchronized with similar signals from the other Omega stations, each of the signals thereby occurring at a unique time in a recurring pattern. Equipped with the proper receiving and interpreting equipment, a navigator may identify the separate source of each received signal within the pattern and determine the relative phase relationship between each pair of such received signals. Points of constant phase difference between these pairs of signals will occur geographically along hyperbolic lines, and Omega navigation charts have been prepared with such hyperbolic lines of position plotted thereon. Using these charts and the phase information from the received Omega signals, a navigator can determine his current location very accurately.
Since the transmitted Omega signals do not provide synchronization or phase reference information, phase calculations at the receiver must be referenced to a local standard. Thus, an Omega receiver is designed to generate its own reference signal at the same frequency as the incoming Omega signal. The appropriate reference signal is then compared with each incoming Omega signal and the phase difference between the reference and the incoming signal is measured. From these preliminary phase measurements, the phase difference between any pair of incoming Omega signals may be calculated.
By transforming the phase information in the incoming Omega signal to binary form, the phase calculations may be conveniently accomplished with digital logic devices. One technique for digitally calculating the desired phase difference begins by filtering the incoming Omega signal, reducing that signal to an intermediate (IF) frequency, and then hard limiting the IF signal. The resulting square wave signal is compared with a reference square wave which is generated by the Omega receiver at the same IF frequency. In order to compare the phase of the Omega signal and the reference signal, the two signals are applied as the inputs of an exclusive-OR logic gate. With these inputs, the exclusive-OR gate will produce a first binary output when both input signals are at the same binary value, while the second binary output will be produced at a time when the input signals are at different binary values. Thus, in effect, the exclusive-OR gate yields the product of multiplying the Omega signal and the reference signal.
The product output of the exclusive-OR gate controls the count direction of an up/down counter. The count which is accumulated in a given time interval will thus be related to the phase difference between the incoming signal and the reference signal, with the lowest count occurring when the signals are in phase, while the highest count will be registered when their phases differ by 180.degree.. The registered count is then used as the error signal in a phase locked loop, which is applied to the reference generator to shift the phase of the reference signal so that it is approximately 90.degree. from the phase of the next incoming Omega signal.
This digital phase detection technique works very satisfactorily for signals which are clean at the input to the hard limiter. For noisier signals, however, the sensitivity of such a phase detector will progressively degrade, and tracking error in the phase locked loop will ultimately build up to as much as one-fourth of a cycle if the phase of the Omega signal is changing. When the tracking error reaches one-fourth cycle, the loop characteristic ceases to be linear, and the loop will unlock, causing a loss of measurement fidelity.
In order to avoid such a phase locked loop error, the technique of quadrature signal processing may be applied to the phase measurement of an Omega signal. In the quadrature design, two phase detector circuits, each of which accomplishes a phase measurement in a manner similar to that of the phase locked loop detector design, are employed. As in the phase locked loop design, the quadrature detector applies a reference signal and the IF Omega signal to the inputs of an exclusive-OR gate.
In the quadrature design, however, two reference signals are generated, with one of the reference signals offset in phase from the other by 90.degree., i.e., the references are produced in quadrature. The Omega signal and the first reference signal are applied to the inputs of a first exclusive-OR gate, while the Omega signal and the 90.degree. phase shifted reference are applied to the inputs of a second exclusive-OR gate. With this arrangement, one up/down counter will accumulate a sum which approximates the sine of the phase difference, while the second up/down counter will register a count which represents the cosine of the phase difference. The phase difference value may then be obtained by calculating the arc tangent of the ratio of these two sums.
In actuality, the sums which are measured are strictly sinusoidal as a function of phase difference only at low signal-to-noise ratios, but approach triangular functions at high signal-to-noise ratios. Thus, the measurements which are obtained may more accurately be referred to as the pseudo-sine and the pseudo-cosine. The approximation, however, is sufficiently close that accurate phase measurements may be obtained by utilizing an appropriate pseudo-arctangent function or even a true arc tangent function. Furthermore, this method of digital phase detection will not degrade in sensitivity as the signal-to-noise ratio decreases, as occurs with the phase locked loop technique discussed above.
The quadrature method of phase detection yields sufficiently accurate results when employed, for navigation purposes, to provide an accurate indication of the location of the Omega receiver. The signals transmitted in the Omega navigation system, however, have proven useful in making additional important measurements. One area in which the Omega signals are potentially of significant utility is meteorology. It is often desirable in the study of weather to chart the speed and direction of the wind over a period of time for various locations about the earth, and at various altitudes over those locations. These measurements may be conveniently obtained using the Omega system.
In order to make such wind finding measurements, instrumented weather balloons are released at diverse locations and allowed to rise through the atmosphere. After a balloon has been released, it will travel with a horizontal component of motion having substantially the same speed and direction as that of the wind at the balloon's location. Instrumentation aboard the balloon transmits its altitude as a function of time. Using the Omega signals, information may also be obtained as to the balloon's horizontal location as a function of time and as to the balloon's speed and direction of travel.
An Omega receiver mounted on the balloon may be designed to retransmit the received signals to a ground station. The ground equipment will then detect phase difference information in a manner similar to that utilized in navigating, as discussed above. Thus the balloon's location may be derived and recorded as a function of time. Furthermore, by detecting the time rate of change of the phase differences between the Omega signals received by the balloon, the speed and direction of travel of the balloon, which may be assumed to correspond to the wind speed and direction at the balloon's location, may be computed. Although the quadrature phase detection technique discussed above produces an arc tangent result which differs by only a few degrees from the phase of the original signal, and although this difference occurs only at high signal-to-noise ratios, this error is detrimental where a highly accurate measurement is required, such as is necessary when computing the time rate of change of the phase difference in the wind finding technique. Consequently, a need has developed in the art for a phase detection device which employs a method which will provide phase difference measurements of increased accuracy.
Therefore, it is a feature of this invention to provide an improved phase detector which is capable of highly accurate measurements of phase difference.
It is another feature of this invention to provide an improved phase detector which is sufficiently accurate to permit the accurate measurement of the time rate of change of phase difference.
It is another feature of this invention to provide an improved phase detector which may be programmed to incorporate digital correction factors into quadrature phase detection circuitry.
It is also a feature of this invention to provide an improved phase detector with which the Omega navigation system may be utilized to perform wind finding measurements.